Foams



Dec. 1 9, 1967 V BLAKEY. JR.. ETALV 3,358,695 7 Y FOAMS Original Filed Dec. 18, 1962 YINVENTORS ERNEST ALBERT COLEMAN II ANGUS RUCKER BLAKEY, JR.

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ATTORNEY United States Patent 3,358,695 FOAMS Angus Rucker Blalrey, Jr., Kennett Square, Pa, and Ernest Albert Coleman ll, Fairfax, DeL, assignors to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Original application Dec. 18, 1962, Ser. No. 245,481, now Patent No. 3,253,967, dated May 31, 1966. Divided and this application July 6, 1965, Ser. No. 478,784

Claims. (Cl. 131269) This application is a division of application Ser. No. 245,481, now Patent 3,253,967, filed Dec. 18, 1962 by Angus Rucker Blakey, Jr., and Ernest Albert Coleman II.

This invention relates to polyoxymethylene foams and their use in forming filters.

In the past, foams of synthetic resins have had closed cell structure at moderate densities. When such foams are crushed or otherwise treated to form an open cell structure, a foam having an irregular distribution of sizes of passages between cells results or the material crushes as in the case of closed cell polystyrene foams. Hence, controlling the performance of these foams in certain uses has been diflicult.

It is an object of this invention to provide a ruptured cell, resilient, low density, thermoplastic, high modulus foam having a uniform distribution of cells of controllable size. A further object of this invention is to provide a ruptured cell foam cigarette filter. Another object is to provide a filter medium suitable for various gas and liquid filtration applications.

These and other objects are accomplished by the following invention in which a polyoxymethylene resin is extruded as a foam by injecting a fluid blowing agent into the resin melt just prior to extrusion (normally in the die end of the extruder). The resin preferably is blended with a nucleating agent prior to extrusion.

The polyoxymethylene resins used in this invention are polymers and copolymers having a structure in which all of the recurring units have the formula:

wherein each of R R and R is selected from the group consisting of hydrogen, lower alkyl, and halogen substituted lower alkyl radicals, and wherein n is an integer of from 0 to 3 and wherein n is 0 in 85 to 100% of the recurring units. These polymers may be prepared by way of several processes for the polymerization of copolyrnerization of formaldehyde, trioxane, or the higher aldehydes. These polymers and processes for their preparation are described in part in U.S. Patents 2,734,889 issued Feb. 14, 1956 to F. C. Starr, Jr.; 2,768,994 issued Oct. 30, 1956 to R. N. MacDonald; 2,828,286 issued Mar. 25, 1959 to R. N. MacDonald; 2,841,570 issued July 1, 1958 to R. N. MacDonald; 2,844,561 issued July 22, 1958 to M. F. Bechtold et al.; 2,848,437 issued Aug, 19, 1958 to W. P. Langsdorf et al.; 2,994,687 issued Aug. 1, 1961 to H. H. Goodman et al.; 3,000,860 issued Sept. 19, 1961 to N. Brown et al.; 3,000,861 issued Sept. 19, 1961 to N. Brown et a1.

One variety of polyoxymethylene, i.e., polyoxymethylene diesters may be prepared by the processes described and claimed in U.S. Patents 2,964,500, issued Dec. 13, 1960 to S. H. Jenkins et al., and 2,998,409, issued Aug. 29, 1961 to S. Dal Nogare et al. Polyoxymethylene diethers may be prepared by the processes described and claimed in copending patent applications Ser. No. 682, 325, filed Sept. 6, 1957 by N. Brown et al., and Ser. No. 785,136, filed Jan. 6, 1959 by N. Brown et al. Included Patented Dec. 19, 1967 within the term polyoxymethylene for purposes of this application are copolymers containing a predominance of oxymethylene groups, such as those prepared by the techniques set forth in U.S. Patents 2,828,287, issued Mar. 25, 1958 to T. L. Cairns et al., 3,027,352, issued Mar. 27,1962 to K. W. Bartz et al., and 3,043,803, issued July 10, 1962 to C. D. Lewis et al., and in copending applications Ser. No. 35,419, filed June 13, 1960 by N. Brown et al.; Ser. No. 35,420, filed June 13, 1960 by N. Brown et al.; Ser. No. 35,421, filed June 13, 1960 by E. T. Cline et al.; Ser. No. 51,294, filed Aug. 23, 1960 by E. T. Cline et al.; Ser. No. 91,318, filed Feb. 24, 1961 by E. T. Cline.

The composition of the final product is primarily a polyoxymethylene resin, but may also include various stabilizers, fillers, colorants, blowing agents, processing aids, and nucleating agents.

Stabilizers may be divided into three types: antioxidants such as phenols, amines, and polypeptides; thermal stabilizers such as polyamides, polypeptides, amidines, amides, and photostabilizers such as benzophenone and derivatives of acrylic acid.

Suitable fillers include high molecular weight synthetic materials that are thermally stable at temperatures up to 250 C. and give neutral or basic decomposition products, natural or synthetic silicates or oxides including glass that exhibit a pH greater than 4 when slurried in distilled water (pH 7) at 1% concentration, metal powders, flakes and shot. Suitable metals include copper, iron, nickel and lead. The fillers may be in the form of powders, flakes, fibers, sheets or foams such as microballoons made of phenolic resins.

Colorants may be used including those which give a sludge pH of 4 or greater at 1% concentration. Useful classes of such colorants include pigments, dyes, pigmentdyes, metal-flakes, fluorescent materials, and carbon.

Processing aids may be used including additives which (a) decrease processing temperatures of the acetal resin, (b) increase or aid in the solubility of the blowing agents in the resin, (c) increase the melt strength or melt elasticity of the resin or (d) decrease or increase friction or stick-slip of the resin melt against the material of construction of the extruder die orifice. Processing aids include aromatic hydrocarbons melting below 200 C. and boiling above 135 C. such as naphthalene and polymeric materials including polystyrene; materials of the same groups as blowing agents, hereinafter described, but melting below 200 C. and boiling above about 135 C. such as benzyl alcohol, glycerine, sulfur, and neutral or basic surface-active agents, such as silicone oil and polyalkene oxide adducts of alkylated phenols.

Suitable nucleating agents preferably are solids of high surface area, preferably 0.28 square meter per gram or greater and which produce more cells in the foam than would be produced in their absence. Examples of these materials include carbon black having a surface area of 0.1 to 30 m. /g., Fiberglas having a surface area of 0.5 to m. /g., and talc, H Mg (SiO having a surface area of 0.01 to 20 m. /g.

Suitable blowing agents are fluids which dissolve in acetal resin at temperatures of 150 to 250 C. at pressures of 1 to atmospheres and have boiling points at atmospheric pressure of less than C. Such blowing agents are selected from the classes consisting of water, ketones, alcohols, aldehydes, esters, halogenated hydrocarbons, nitriles, gases, such as carbon dioxide, nitrogen, ammonia, and formaldehyde and combinations thereof. Included in these classes are aqueous solutions of formaldehyde, aqueous solutions of ammonia (ammonium hydroxide) and aqueous solutions of methyl ethyl ketone. Also included in these classes are dichlorodifluoromethane, dichloromethane, chloroform, fluorotrichloromethane, 1,1,1-trichloroethane, 1,1,2-trifluoroperchl0roethane,

ethanol butanol-Z, ethyl acetate, acetaldehyde, ethyl chloride and acetonitrile. Of these, water, methyl ethyl ketone, methylene chloride, chloroform, methyl ethyl ketone in water, and chloroform in dichloromethane are preferred with water being especially preferred. Water is especially preferred because it has a very high capacity for absorbing heat which causes rapid freezing of the melt and an especially uniform sized rupture of the cells.

It is pointed out that some materials may serve more than one function. For instance, some nucleating agents may also serve as fillers or colorants, just as some processing aids also double as blowing agents.

The foams preferably have surface areas, as measured by the Brunauer-Emmett-Teller (B-E-T) nitrogen-adsorption method, greater than 0.4 m. /gm., but less than 4.0 mF/gm. The density of the foams preferably is between 0.001 gm./cc. and 0.320 gm./cc. Wall thickness can be measured directly with a micrometer, or a single wall can be analyzed for thickness by determining if X- rays or an electron beam will pass through it. Walls over about 2 microns are opaque to X-rays and electron beams. Wall thickness of from 2 microns to 100 microns are preferred. The cells are preferably smaller than 1 mm. across.

These polyoxymethylenes have been found to make excellent cigarette filters, having the advantages of low cost, ease of fabrication, low pressure drop, resiliency, a high surface area; they are odorless, tasteless, white, and have high filter efiiciency. By way of comparison, 50* to 80 mg. of polyoxyrnethylene foam removes as much tar and nicotine as 90 to 140 mg. of cellulose acetate in the commercial cigarette filters tested.

Cigarette filters can be made by passing the extruded foam in ribbon form under a wire brush running over a metal plate to shred the foam into portions /2 to 3 inches in length with a maximum diameter of about inch. The shreds are then assembled into a filter by the same technique by which tobacco is normally wrapped with paper to form cigarettes. A preferred method of making filters is to wrap the extruded foam ribbon with paper.

In the drawing: FIGURE 1 is a cross section of an extruder used for extruding foams.

FIGURE 2 is a side view of the foam shredder.

FIGURE 3 is a view of the shreds being wrapped.

FIGURE 4 is a view of the completed filter.

The polyoxymethylene resin 1 is fed into the feed hopper 2 of extruder 3, maintained at from 150 to 200 C., and screw 4 rotated at a speed sufficient to give a pressure in excess of 200 p.s.i. at the die 6. The blowing agent is injected under pressure thru probe 5, and the TABLE I.-C ON DITIONS FOR The following examples are illustrative of the invention.

Example 1 A 2-inch extruder as shown in FIGURE 1 is used, with the injection probe 6 inches from the die slit which measures 0.115 by 1 inch. The extruder screw is driven by a motor at screw speeds varying from to rpm, and a constant barrel pressure of about 4500 psi. A polyoxymethylene diacetate having a broad molecular weight distribution of 20,000100,000 made by blending resins of different molecular weights containing 1% by weight of a terpolymer of 38% and polycaprolactam/ 35 polyhexamethylene adipamide/ 27%, polyhexamethylene sebacamide and 0.1% by weight of 4,4'-butylidene bis(4- methyl-G-tertiary-butyl phenol) mixed with 2% Fiberglas having a diameter of 0.003 and a length of and 1% naphthalene is fed into the extruder. The output of the extruder varied from 50 to 80 lbs./hr., and S to 30 lbs/hr. of water was injected as the blowing agent. The melt in the barrel was maintained at the following temperatures at the locations indicated in FIGURE 1, T 210 C., T 200 C., T 185 C., T, 180 C., T 180 C., T 180 C. The resulting foam had a density of 0.03 g./cc., and an average cell size of 0.5 mm., which ranged from 0.1 to 1.0 mm. of which over 80% were open cells and less than 20% were closed cells as judged by microscopic examination of cell walls.

Example 1 was repeated using the same apparatus but varying the conditions and the materials used. The results are reported in Examples 2 to 4 set forth in Table I. In Example 3, a four-hole inch beading die was substituted for the dye used in Examples 1, 2 and 4. In Example 4, a 4" diameter radial foamed film die was used. The polymer used in Example 2 was a copolyrner or approximately 98.5 mole percent --CH O units and 1.5 mole percent CH CH O- units having an average molecular weight of about 35,000. The polymer used in Example 3 was a blend of Va polyoxymethylene diacetate having an average molecular weight of 60,000 and /5 polyoxymethylene diacetate having an average molecular weight of 35,000. Example 4 used a blend of 50% polyoxymethylene diacetate having an average molecular weight of 6 0,000 and 50% of a polyoxymethylene diacetate having an average molecular weight of 35,000. The polymer used in each of Examples 3 and 4 contained 1% of a terpolymer of 38% polycaprolactam/ 35 polyhexamethylene adipamide and 27% polyhexamethylene sebacamide and 1% of 4,4'-butylidene bis(4-methyl-6-tertiary butyl phenol).

The properties of the foams made in Examples 2 to 4 are reported in Table II.

FOAMIN G POLYOXYMETHLYENES Temperature Pressure Rate Screw Ex. Blowing Agent Additives speed,

T T T T T Tu, r.p.m. Barrel, Die, Output, Blowing C. C. 0. 0. 0. C. p.s.i. p.s.i. lbs/hr. Agent, lbs/hr.

2-. 210 200 170 165 165 0 vols. methyl/ethyl 5% glass fiber 30-45 1,500 50 25 3 ketone, 1 vol. chloroform. 3 215 195 185 185 185 Water 5% glass fiber, 5% 70 3,000 500 45 7 olystyrene. 4 215 195 185 185 140 Methyl/ethyl 2 0 na hthalene, 60-80 1,000 200 45 r ketonefwater 1;2. 0 5% glass fiber.

TABLE IL-STRUCTURE OF POLYOXYMETHYLENE FOAMS Density, Average Ruptured Closed Example gm./cc. Cell Size, Cells, Cells,

average mm. Percent Percent The foam produced in Example 1 is continuously passed under a rotating 3" diameter wire brush 3 inches wide made of 12 inch diameter steel wires, to cut the foam into shreds with a maximum diameter of about /s inch, averaging 1 inch in length, but varying from /2 to 3%. inches in length. The shreds are then rolled in cigarette papers to form a long filter which is then cut into 4 inch lengths and attached to various brands of filter cigarettes to replace the original filter and tested. The test results of this and various other polyoxymethylene foam filters are reported in Table III. The test used was the Cambridge Filter Disc Method in which a glass filter disc is inserted in the smoke train from an artificially smoked cigarette. The efficiency of the cigarette filter is then ascertained by the weight of material caught by the glass filter disc. The pressure drop across the cigarette filter is ascertained before test smoking by a manometer filled with water with one end connected to the smoke train line and the other end open to the atmosphere so as to measure the pressure drop across the filter in inches of water. An air flow rate of 1050 cc./min. was used to measure pressure drop. A two-second puff duration at 35 mL/puff is used in all the smoking tests. The filter disc was dried (1 hr. at 105 C.) before test smoking to avoid errors due to residual moisture on the glass filter. Runs 1 to 6 each used the tobacco end from the same brand of cigarette and smoked 49 mm. of cigarette. Runs 7 to 11 each used the tobacco end from the same brand of cigarette and smoked 46 mm. of cigarette. Run 5 used a filter made by wrapping 3 ribbons of the foam extruded in Example 1 with paper and then cutting to 15 mm. lengths. Run 6 used a filter made by wrapping 4 ribbons of the foam of Example 1 with paper and then cutting to 15 mm. lengths. Runs 2 to 4 and 8 to 11 used the foam filters shredded by means of a wire brush as described above.

agent upon extrusion. The procedure of this invention leaves residual cell walls which creates a large surface area while giving a good flow due to the large percentage of uniformly ruptured cells.

The extruded foams of this invention are useful as filter elements in a wide variety of applications. These applications include air conditioner filters, and solid-liquid separation filters such as automotive oil filter elements and dry cleaning solvent filters. The solid, as extruded foam can be used in these applications, and it has sulficient strength and resiliency that little, if any, support is needed for such filter elements when used in low pressure applications.

We claim:

1. A cigarette filter element formed of a predominantly ruptured celled polyoxymethylene foam having an aver age cell size of under 1 millimeter, a surface area of from 0.4 to 4.0 square meters per gram, a density of from 0.001 to 0.320 gram per cubic centimeter, and an average wall thickness of over 2 microns.

2. A cigarette filter formed of shreds of up to A; inch average diameter formed of a predominantly ruptured celled polyoxym'ethylene foam having an average cell size of under 1 millimeter, a surface area of from 0.4 to 4.0 square meters per gram, a density of from 0.001 to 0.320 gram per cubic centimeter, and an average wall thickness of over 2 microns.

3. A cigarette filter formed of shreds of up to inch average diameter formed of a predominantly ruptured celled polyoxymethylene foam having a surface area of from 0.4 to 4.0 square meters per gram, a density of from 0.001 to 0.320 gram per cubic centimeter, and an average wall thickness of over 2 microns.

4. A cigarette filter formed of a plurality of ribbons of a predominantly ruptured celled polyoxymethylene TABLE IIL-PERFORMANCE OF POLYOXYMETHYLENE FOAMS IN CIGARETTE FILTERS Pressure Material Wt. of Wt. Filter Drop in Passed per Run Filter Cigarette Materials Inches Water Cigarette (mg) (mg.) on Cigarette (mg) Plus Filter 1 Commercial Cellulose Acetate #1 838 130 2.0 24 2 Foam of Example 1 845 69 1. 1 19 3 do 80 1. 3 21 4 55 1. 7 24 5 70 0.8 19 6 68 0.7 10 7 93 1. 4 25 8 93 l. 5 22 9 56 3.0 19 59 4. 5 13 32 2. 1 24 As can be seen from Table III, the polyoxymethylene foam of this invention is more efiicient than cellulose acetate for filtering tobacco smoke on a weight basis.

Other methods may be used to form cigarette filters. For example, chopped foam may be wrapped as abovedisclosed for the shreds. Alternatively, a cylindrical shape predominantly open cell foam may be cut the length of the desired cigarette filter, or the foam ribbon from which shreds are made can be Wrapped in paper to make cigarette filters. Both the chopped foam and cylindrical methods produce effective filters although they are not as eificient as the shredded filter or the foam ribbon type filters.

The foams of this invention have structures which are at least and preferably 75% ruptured celled. Other materials which are formed into open cell foams are formed first as closed cell foams and then crushed or otherwise mechanically treated to break the cell walls, rather than having the cell walls ruptured by the blowing UNITED STATES PATENTS 3,006,346 10/1961 Golding 131-10.7 3,120,849 2/1964 Guttag 131-109 SAMUEL KOREN, Primary Examiner. D. J. DONOHUE, Assistant Examiner. 

1. A CIGARETTE FILTER ELEMENT FORMED OF A PREDOMINANTLY RUPTURED CELLED POLYOXYMETHYLENE FOAM HAVING AN AVERAGE CELL SIZE OF UNDER 1 MILLIMETER, A SURFACE AREA OF FROM 0.4 TO 4.0 SQUARE METERS PER GRAM, A DENSITY OF FROM 0.001 TO 0.320 GRAM PER CUBIC CENTIMETER, AND AN AVERAGE WALL THICKNESS OF OVER 2 MICRONS. 